The parenchyma tissue is composed of living cells having thin walls and a polyhedral shape, and concerned with vegetative activities of the plant. The parenchyma is the primitive tissue since the lower plants have given rise to the higher plants through specialization and since the single type or the few type of cells found in the lower plants have become by specialization the many and elaborate types of the higher plants.
The parenchyma consists of isodiametric, thin walled and equally expanded cells. The parenchyma cells are oval, rounded or polygonal in shape having well developed space among them. The cells are not greatly elongated in any direction. The cells of this tissue are living and contain sufficient amount of cytoplasm in them. Usually each cell possesses one or more nuclei.
Parenchyma makes up large parts of various organs in a plant. Pith, mesophyll of leaves, the pulp of fruits, endosperm of seeds, cortex of stems and roots, and other organs of plants consist mainly of parenchyma. The parenchyma cells also occur in xylem and phloem.
In aquatic plants, the parenchyma cells in the cortex possess well developed air spaces and are called aerenchyma.The turgid
parenchyma cells help in giving rigidity to the plant body. Partial conduction of water is also maintained through parenchymatous cells. The parenchyma cells acts as special storage tissue to store food materials in the form of starch grains, proteins, fats and oils. The parenchyma cells which contain chloroplasts in them make chlorenchyma which is responsible for photosynthesis in green plants. In water plants the parenchyma keep up the buoyancy of the plants.